A typical inkjet printer reproduces an image by ejecting small drops of ink from a printhead containing nozzles, where the ink drops land on a receiver medium (typically paper) to form ink dots. A typical inkjet printer reproduces a color image by using a set of color inks, usually cyan, magenta, yellow, and black. It is well known in the field of inkjet printing that if ink drops placed at neighboring locations on the page are printed at the same time, then the ink drops tend to flow together on the surface of the page before they soak into the page. This can give the reproduced image an undesirable grainy or noisy appearance often referred to as “coalescence”. It is known that the amount of coalescence present in the printed image is related to the amount of time that elapses between printing adjacent dots. As the time delay between printing adjacent dots increases, the amount of coalescence decreases, thereby improving the image quality. There are many techniques present in the prior art that describe methods of increasing the time delay between printing adjacent dots using methods referred to as “interlacing”, “print masking”, or “multipass printing”. There are also techniques present in the prior art for reducing one-dimensional periodic artifacts referred to as “bands” or “banding.” This is achieved by advancing the paper by an increment less than the printhead width, so that successive passes or “swaths” of the printhead overlap. The techniques of print masking and swath overlapping are typically combined. See, for example, U.S. Pat. Nos. 4,967,203 and 5,992,962. The term “print masking” generically means printing subsets of the image pixels in multiple partially overlapping passes of the printhead relative to a receiver medium.
Another attribute of modem inkjet printers is that they typically possess the ability to vary (over some range) the amount of each ink that is deposited at a given location on the page. Inkjet printers with this capability are referred to as “multitone” inkjet printers because they can produce multiple density tones at each location on the page. Some multitone inkjet printers achieve this by varying the volume of the ink drop produced by the nozzle by changing the electrical signals sent to the nozzle or by varying the diameter of the nozzle. See for example U.S. Pat. No. 4,746,935. Other multitone inkjet printers produce a variable number of smaller, fixed size droplets that are ejected by the nozzle, all of which are intended to merge together and land at the same location on the page. See for example U.S. Pat. No. 5,416,612. These techniques allow the printer to vary the size or optical density of a given ink dot, which produces a range of density levels at each location, thereby improving the image quality.
Another common way for a multitone inkjet printer to achieve multiple density levels is to print a small amount of ink at a given location on several different passes of the printhead over that location. This results in the ability to produce a greater number of density levels than the nozzle can fundamentally eject, due to the build up of ink at the given location over several passes. See, for example, U.S. Pat. No. 5,923,349.
In U.S. Pat. No. 5,790,150, Lidke et al. disclose a method where multiple passes are made over the page before the page is advanced. In each pass, the pattern of dots in the data swath is constructed with sufficient spacing between the dots such that the printhead can be scanned across the page at a velocity that is higher than the firing frequency limit of the nozzles.
In U.S. Pat. No. 6,206,502, Kato et al. disclose a print masking method in which nozzles at the ends of the printhead print with lower duty than nozzles near the center of the printhead, thereby reducing the possibility of banding artifacts occurring at the boundaries between successive printed swaths.
In U.S. Pat. No. 6,238,037, Overall et al. disclose a print masking method for a multilevel inkjet printer in which the print mask contains a set of threshold values. A dot will print at a given location on a given pass if the multitone code value for that pixel is greater than the threshold for that pass. This method requires that if a dot gets printed at a given pixel on pass N, then it also must receive dots on passes 0 through N-1.
In U.S. Pat. No. 6,454,389, Couwenhoven et al. disclose a print masking method suitable for multilevel inkjet printers that can produce multiple sized ink drops.
In all of the above mentioned inkjet printers, the designer of the printer is faced with the task of splitting the image data into multiple memory buffers corresponding to the multiple passes of the printhead. It is believed that the prior art methods are constrained so that the dot patterns printed corresponding to one multitone level are highly correlated with the dot patterns printed corresponding to another multitone level. This restriction can lead to undesirable print artifacts or excessive or unbalanced use of some nozzles. Therefore there is a need for improvement over the prior art in the area of multipass printing to support multitone ink jet printers which eject multiple drops at a given location on several different passes.